Many plastic articles are formed by thermoforming molding processes. These articles include, for example, common containers such as Jell-O® cups, yogurt cups, fruit cups and similar containers having (1) a body portion which has a closed base, side walls coupled to the base and an open terminal end and (2) a protrusion such as a flange portion or lip portion extending radially outward or inward from the side walls. In the art, these containers are commonly produced by thermoforming processes.
Due to the geometries of a mold cavity needed to form a container with a lip, flange or any other type of protrusion, it is difficult to control the downstream flow of the interior core stream and selectively direct the interior core stream into a downstream branch channel branching from a branch junction of a main channel of the mold cavity. It is therefore desirable to have a process for the formation of these flanged articles by co-injection molding whereby the interior core stream can be controlled such that (1) it is selectively directed along a desired pathway in the mold cavity, (2) directed as close to a terminal end of a branch channel that defines a protrusion of the resulting plastic article (3) while simultaneously avoiding the breakthrough of the combined flow-front by the interior core stream.
A further problem experienced when injection molding such articles is that a hole or gap is formed in the interior core layer at the base of the molded article where the interior core layer enters the mold. The hole is formed because the interior core stream is formed by an annular stream with a diameter that decreases towards the base of the molded article. The diameter of the annular core stream at the base of the article corresponds directly to the diameter of the hole or gap. In particular, the interior core layer enters the mold as an annular stream which is surrounded on both sides by inner and outer skin layers. When the flow of the interior core layer is stopped, a tail of the interior core layer continues up the sidewall of the molded part, thereby creating a hole at the base of the molded part which is typically much larger than a gate of the nozzle which injects the interior core and outer layers.
Controlling the size of the hole or gap created by the annular interior core streams is fundamental in present day injection molding systems. If this gap is too large, the barrier properties of the molded part will be significantly reduced. In other words, a vacuum created within a substance-containing portion of the part cannot be maintained for a long period of time because exterior gases will enter the part through the hole, or conversely pressure can not be maintained in the part because gases within the molded part will seep out through the hole. Thus, there is presently a need for a method and apparatus for injection molding articles where the size of a gap or hole in the interior plastic layer is efficiently controlled.